/****************************************************************************
* Function : AssignCanonicalCode
* Description: This function accepts a list of symbols sorted by their
* code lengths, and assigns a canonical Huffman code to each
* symbol.
* Parameters : cl - sorted list of symbols to have code values assigned
* Effects : cl stores a list of canonical codes sorted by the length
* of the code used to encode the symbol.
* Returned : TRUE for success, FALSE for failure
****************************************************************************/
static int AssignCanonicalCodes(canonical_list_t *cl)
{
int i;
byte_t length;
bit_array_t *code;
/* assign the new codes */
code = BitArrayCreate(256);
BitArrayClearAll(code);
length = cl[(NUM_CHARS - 1)].codeLen;
for(i = (NUM_CHARS - 1); i >= 0; i--)
{
/* bail if we hit a zero len code */
if (cl[i].codeLen == 0)
{
break;
}
/* adjust code if this length is shorter than the previous */
if (cl[i].codeLen < length)
{
BitArrayShiftRight(code, (length - cl[i].codeLen));
length = cl[i].codeLen;
}
/* assign left justified code */
if ((cl[i].code = BitArrayDuplicate(code)) == NULL)
{
perror("Duplicating code");
BitArrayDestroy(code);
return FALSE;
}
BitArrayShiftLeft(cl[i].code, 256 - length);
BitArrayIncrement(code);
}
BitArrayDestroy(code);
return TRUE;
}
/***************************************************************************
* Function : BitArrayDuplicate
* Description: This function duplicates (creates and copies) the bit array
* passed as a parameter and returns a pointer to the
* duplicate.
* Parameters : src - pointer to bit array to be duplicated
* Effects : A duplicate of the source bit array is created.
* Returned : Pointer to duplicate of source or NULL on failure.
***************************************************************************/
bit_array_t *BitArrayDuplicate(const bit_array_t *src)
{
bit_array_t *ba;
if (src == NULL)
{
return NULL; /* no source array */
}
ba = BitArrayCreate(src->numBits);
if (ba != NULL)
{
ba->numBits = src->numBits;
BitArrayCopy(ba, src);
}
return ba;
}
/****************************************************************************
* Function : MakeCodeList
* Description: This function uses a huffman tree to build a list of codes
* and their length for each encoded symbol. This simplifies
* the encoding process. Instead of traversing a tree in
* search of the code for any symbol, the code maybe found
* by accessing codeList[symbol].code.
* Parameters : ht - pointer to root of huffman tree
* codeList - code list to populate.
* Effects : Code values are filled in for symbols in a code list.
* Returned : 0 for success, -1 for failure. errno will be set in the
* event of a failure.
****************************************************************************/
static int MakeCodeList(huffman_node_t *ht, code_list_t *codeList)
{
bit_array_t *code;
byte_t depth = 0;
if((code = BitArrayCreate(EOF_CHAR)) == NULL)
{
perror("Unable to allocate bit array");
return -1;
}
BitArrayClearAll(code);
for(;;)
{
/* follow this branch all the way left */
while (ht->left != NULL)
{
BitArrayShiftLeft(code, 1);
ht = ht->left;
depth++;
}
if (ht->value != COMPOSITE_NODE)
{
/* enter results in list */
codeList[ht->value].codeLen = depth;
codeList[ht->value].code = BitArrayDuplicate(code);
if (codeList[ht->value].code == NULL)
{
perror("Unable to allocate bit array");
BitArrayDestroy(code);
return -1;
}
/* now left justify code */
BitArrayShiftLeft(codeList[ht->value].code, EOF_CHAR - depth);
}
while (ht->parent != NULL)
{
if (ht != ht->parent->right)
{
/* try the parent's right */
BitArraySetBit(code, (EOF_CHAR - 1));
ht = ht->parent->right;
break;
}
else
{
/* parent's right tried, go up one level yet */
depth--;
BitArrayShiftRight(code, 1);
ht = ht->parent;
}
}
if (ht->parent == NULL)
{
/* we're at the top with nowhere to go */
break;
}
}
BitArrayDestroy(code);
return 0;
}
int CHuffmanInit(struct CHuffman *ch)
{
int i;
/* initialize canonical list */
for (i = 0; i < NUM_CHARS; i++) {
ch->canonicalList[i].codeLen = 0;
ch->canonicalList[i].code = NULL;
}
if (!ReadHeader(ch->canonicalList)) {
return FALSE;
}
/* sort the header by code length */
qsort(ch->canonicalList, NUM_CHARS, sizeof(canonical_list_t),
CompareByCodeLen);
if (AssignCanonicalCodes(ch->canonicalList) == 0)
{
/* failed to assign codes */
for (i = 0; i < NUM_CHARS; i++)
{
if(ch->canonicalList[i].code != NULL)
{
BitArrayDestroy(ch->canonicalList[i].code);
}
}
return FALSE;
}
if (ch->for_encode) {
/* re-sort list in lexical order for use by encode algorithm */
qsort(ch->canonicalList, NUM_CHARS, sizeof(canonical_list_t),
CompareBySymbolValue);
}
else {
/* decoding */
/* allocate canonical code list */
ch->code = BitArrayCreate(256);
if (ch->code == NULL) {
perror("Bit array allocation");
return FALSE;
}
ch->resume = 0;
/* create an index of first code at each possible length */
for (i = 0; i < NUM_CHARS; i++)
{
ch->lenIndex[i] = NUM_CHARS;
}
for (i = 0; i < NUM_CHARS; i++)
{
if (ch->lenIndex[ch->canonicalList[i].codeLen] > i)
{
/* first occurance of this code length */
ch->lenIndex[ch->canonicalList[i].codeLen] = i;
}
}
}
return TRUE;
}
